1. Technical Field
The present application relates to a GaN-based semiconductor light-emitting element such as a light-emitting diode or a laser diode.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element, because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors including Ga as a Group III element (which will be referred to herein as “GaN-based semiconductors” and which are represented by the formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1)) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c of a wurtzite crystal structure. The primitive vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. Furthermore, a plane which is terminated with a Group III element such as Ga is called either a “+c plane” or a “(0001) plane”, while a plane which is terminated with a Group V element such as nitrogen is called either a “−c plane” or a “(000-1) plane”. That is to say, these two crystal planes are dealt with as different ones. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”.
In fabricating a semiconductor element using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which GaN semiconductor crystals will be grown. In a c plane, however, Ga atoms and nitrogen atoms do not exist on the same atomic plane, thus producing electrical polarization there. That is why the c plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers. Consequently, the internal quantum efficiency decreases, thus increasing the threshold current in a semiconductor laser diode and increasing the power consumption and decreasing the luminous efficiency in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate, of which the principal surface is a non-polar plane such as a (10-10) plane that is perpendicular to the [10-10] direction and that is called an “m plane”, be used (i.e., an m-plane GaN substrate be used). In this description, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m plane is parallel to the c axis and intersects with the c plane at right angles. On the m plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no spontaneous electrical polarization will be produced perpendicularly to the m plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above. In this case, the “m plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
In this description, the “a plane” refers herein to a (11-20) plane, which intersects with the [11-20] direction at right angles. As shown in FIG. 3C, the a plane is parallel to the c axis (i.e., the primitive vector c) and intersects with the c plane at right angles. In this case, the “a plane” is a generic term that collectively refers to a family of planes including (11-20), (−1-120), (1-210), (−12-10), (−2110) and (2-1-10) planes.
In this description, the “+r plane” refers herein to a (10-12) plane, which intersects with the [10-12] direction at right angles. The r plane is shown in FIG. 3D. In this case, the “+r plane” is a generic term that collectively refers to a family of planes including (10-12), (−1012), (1-102), (−1102), (01-12) and (0-112) planes.
In this description, the “−r plane” refers herein to a (10-1-2) plane, which intersects with the [10-1-2] direction at right angles. In this case, the “−r plane” is a generic term that collectively refers to a family of planes including (10-1-2), (−101-2), (1-10-2), (−110-2), (01-1-2) and (0-11-2) planes.
Meanwhile, some people devised a method for transferring the nanostructure of a film onto the surface of a semiconductor light-emitting element by covering the surface of the semiconductor light-emitting element with such a film and dry-etching the surface using that film as a photolithographic mask. For example, Japanese Laid-Open Patent Publication No. 2009-94219 discloses a method for transferring a nanostructure using nanoparticles as an etching mask. Japanese Laid-Open Patent Publication No. 2009-302578 discloses a method for transferring a nanostructure using block copolymers as an etching mask. And Japanese Laid-Open Patent Publication No. 2009-225787 discloses a method for transferring a nanostructure using metallic nanoparticles as an etching mask.